1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, a copying machine or a facsimile of a contact charging system in which an image bearing member such as an electrophotographic photosensitive body or an electrographic recording dielectric is uniformly charged with a predetermined polarity and potential.
2. Related Art
An explanation will be made giving an example of an image forming apparatus utilizing an electrophotographic system. Conventionally, there has been an image forming apparatus called xe2x80x9ccleaning simultaneous with developingxe2x80x9d or xe2x80x9ccleanerlessxe2x80x9d in which arrangement of a cleaning means as a dedicated device is eliminated by making a developing means also serve as a cleaning means for toner remaining on a surface of a photosensitive drum (hereinafter referred to as transfer residual toner) after a toner image transfer with respect to a transferring material for attaining the effect of reducing the whole apparatus in size, dealing with ecology with no occurrence of waste toner, the effect of lengthening life of an electrophotographic photosensitive body (hereinafter referred to as photosensitive drum) that is an image bearing member, and reduction of a consumption amount of toner that is a developer per one page.
There is disclosed in, for example, U.S. Pat. No. 6,128,456 the above-mentioned image forming apparatus, which is sometimes called xe2x80x9ccleaning simultaneous with developingxe2x80x9d or a xe2x80x9ccleanerlessxe2x80x9d apparatus in which a charging means of a direct charging system with no use of discharge is used as a charging process means of the photosensitive drum.
In this image forming apparatus, conductive charge accelerating particles are interposed in a contact portion of a contact charging means (contact charging member) contacting the photosensitive drum and the photosensitive drum, and only a direct current voltage is applied, thereby obtaining a surface potential of the photosensitive drum which is substantially equal to the applied direct current voltage.
This system does not positively use discharge, and thus, there is no occurrence of ozone. Further, in this system, adhesion of a discharge product to the photosensitive drum can be suppressed, and the problem of smeared image (image flow) and the like under high temperature and high humidity environment is prevented.
However, in the above-mentioned image forming apparatus of the cleaning simultaneous with developing system which is provided with the charging means using the direct charging system has had the following problem.
In case of using the direct charging system charging means disclosed in U.S. Pat. No. 6,128,456, there is a case where, as the number of sheets to be passed increases, the transfer residual toner stuck to the contact charging means is accumulated, and thus, the contact charging means is contaminated, which leads to charging defect that causes image defect.
This phenomenon occurs because the transfer residual toner stuck to the contact charging means is not electrostatically expelled since the direct current voltage applied to the contact charging means and the charging potential of the surface of the photosensitive drum are substantially equal to each other. In particular, in the case where the structure is adopted in which the transfer residual toner is positively scraped from the photosensitive drum by providing speed difference between the photosensitive drum and the contact charging means, the above-mentioned image defect is conspicuous.
The present inventors make a proposal for the method of solving the above problem while making use of the merit of direct charging in which discharge is not positively used in U.S. patent application Ser. No. 09/921,700.
This proposal is characterized in that cleaning supporting particles as conductive particles with the purpose of accelerating charging are interposed in a contact portion of a contact charging means (charging cleaning member) and a photosensitive drum that is an image bearing member, and that an alternating voltage with a peak to peak voltage of 500 V or higher and below a charging potential convergence voltage is superimposed on and applied to a direct current voltage at the time of image formation.
In more detail, the cleaning supporting particles having a particle resistance of 1012 xcexa9xc2x7cm or less are interposed in the contact portion of the contact charging means, which charges the photosensitive drum as the image bearing member, which has flexibility, and of which at least an outer peripheral surface is porous, and the photosensitive drum, and the alternating voltage is superimposed and applied at the time of image formation. Thus, the contact charging means scrapes the toner that is the developer remaining on the photosensitive drum after transfer. Also, the toner stuck to the contact charging means does not become to have positive polarity since it is not influenced by discharge, and the toner does not remain on the contact charging means and can be expelled to the downstream side in a rotational direction of the photosensitive drum due to the potential difference between the contact charging means and the surface of the photosensitive drum.
Further, in the case where only the direct current voltage is applied, the surface of the photosensitive drum is uniformly charged without unevenness at a predetermined potential, with the result that charging in which direct injection charging is dominant can be realized.
Here, the contact charging means is driven with the speed difference with respect to the photosensitive drum. Thus, the scraping property of the residual toner is improved, and the stable charging property can be obtained.
Furthermore, the above is combined with an action of the cleaning supporting particles with a particle size of 0.1 to 3 xcexcm which exist on the contact charging means. Thus, the toner stuck to the contact charging means can be effectively expelled.
Therefore, by taking the structure proposed in U.S. patent application Ser. No. 09/921,700, an alternating electric field suitable for expelling the toner that is the developer from the contact charging means is formed. Thus, it is possible to prevent contamination of the contact charging means over a long period of time, thereby obtaining the stable charging property.
On the other hand, cleaning simultaneous with developing is conducted by using a contact developing means in which a developer carrying member contacts the photosensitive drum. Thus, output images with high quality and with little fog can be obtained over a long period of time without an influence of deflection of the charging potential. Moreover, the transfer efficiency is improved by using spherical toner, and thus, the toner remaining on the photosensitive drum after transfer can be reduced. At the same time, expelling at the time when the toner is stuck to the contact charging means becomes easy. The proposal is made such that stabilization of a cleanerless system can be attained over a long period of time.
However, in the proposal in U.S. patent application Ser. No. 09/921,700, it is found out that the following disadvantage is seen in the case where specific pattern images are continuously formed particularly in low temperature and low humidity environment.
As described above, when the alternating voltage is superimposed and applied at the time of image formation, the porous contact charging means expels the scraped transfer residual toner to the downstream side in the rotational direction of the photosensitive drum simultaneously with the cleaning supporting particles interposed in the contact portion of the photosensitive drum and the contact charging means.
The cleaning supporting particles are interposed in the contact nip portion of the photosensitive drum and the contact charging means, and are low resistance particles essential for stably maintaining the charging in which direct injection charging is dominant. The particles are interposed in the nip portion even if the contact charging means is somewhat contaminated by the toner. Thus, the surface of the photosensitive drum can be uniformly charged without unevenness at a predetermined potential. However, in the case where the amount of interposal of the particles becomes a predetermined amount or less with respect to the amount of contamination due to toner, it is difficult to secure charging uniformity.
In particular, when images of a longitudinal belt pattern or the like are continuously formed in the low temperature and low humidity environment in which securing charging property is difficult, the transfer residual toner is continuously scraped at a specific part of the contact charging means in a longitudinal direction. Thus, there is a problem in that the amount of interposal of the particles is relatively few with respect to the amount of contamination due to the toner in the part.
This problem occurs similarly in the case where the amount of transfer residual toner that is scraped by the contact charging means exceeds a predetermined amount, for example, in the case where the transfer efficiency is conspicuously poor under a certain condition, or in the case where a jam is occasioned at the time of image formation with a high printing ratio.
Namely, the alternating voltage is superimposed and applied to the contact charging means at the time of image formation, whereby the scraped transfer residual toner can be expelled to the downstream side in the rotational direction of the photosensitive drum. Thus, the toner is not accumulated on the contact charging means. However, the cleaning supporting particles interposed in the nip portion of the photosensitive drum and the contact charging means are also expelled, and the charging uniformity can not be secured if a plurality of bad conditions are present.
The present invention has been made in view of the above-described problem, and an object of the present invention is therefore to provide an image forming apparatus in which the effect of enlarging the apparatus and the increase in cost are suppressed.
Another object of the present invention is to provide an image forming apparatus in which conductive particles are prevented from separating from a charging member to an image bearing member.
Still another object of the present invention is to provide an image forming apparatus in which charging unevenness is prevented and a developer stuck to a charging member is easily expelled to an image bearing member.
Yet still another object of the present invention is to provide an image forming apparatus in which charging can be conducted stably to obtain satisfactory images without occurrence of charging defect over a long period of time.